LIGHT DISCRIMINATION IN THE ENGLISH SPARROW 91 



human subjects. There seems to be little doubt that for the 

 particular intensity the human subjects have a very much 

 smaller threshold of discrimination than do the birds. However, 

 Male IV could discriminate almost as fine a difference in intensity 

 as did two of the men. When the difference between the two 

 lights became very small two of the subjects found that they 

 were simply guessing and not really discriminating. Yet their 

 results appear very good. So even from the results it is very 

 difficult to determine just what their thresholds of discrimina- 

 tion are. 



(2) Methods of learning. — The data which this investigation 

 gives regarding the learning process jof sparrows is fully as im- 

 portant as the light which it throws on the threshold of visual 

 discrimination. More problems were of course raised than were 

 answered, but much light is thrown upon many of the factors 

 which condition animal learning and which in turn condition 

 the sensory threshold which an animal may acquire. One of 

 the most striking facts is the very large number of trials neces- 

 sary to bring the animal to the threshold. The three animals 

 for which the threshold was determined averaged 2420 trials 

 each. For the discrimination of the lowest threshold they aver- 

 aged 480 trials each; one of them discriminating only after 615 

 trials. This bird was trained daily, Sunday excepted, from 

 April 8th until May 31 before she made a record of perfect 

 choices for two days in succession. 



(a) Tables and results for each bird. The results for each 

 bird were tabulated in a regular form which stated the intensity 

 (in Hefner units) of both the standard and the variable lights 

 for each separate position; the difference between the intensity 

 of the standard and the intensity of the variable; the date of 

 each series; the number of each series; the number of right 

 choices; the number of wrong choices; the number of times the 

 wrong choices occurred when the standard was on the left; the 

 number of times the wrong choices occurred when the standard 

 was on the right; and the percentage of error for each series. 



Male IV was allowed to choose the brighter of the two illumi- 

 nated areas. The intensity of the standard was .098 c.p. and at 

 the beginning of the experiment the intensity of the variable 

 was .002 c.p. After each successful series of 30 choices the light 

 was shifted. 



